When Hope Litoff’s sister Ruth, a talented photographer and artist, killed herself, her belongings were put in storage because they were “too painful to look at and too important to throw away”.

Six years on and still struggling to understand why Ruth took her life, Hope decided to search for answers in the journals, piles of artworks, and boxes filled with bottles of prescription pills and more mundane items that were gathering dust in the lock-up. She filmed the process, including her emotional unravelling and return to alcoholism as she confronted her grief.

The result is the raw and brutally candid documentary, 32 Pills: My Sister’s Suicide. The documentary has been shown at several film festivals and will be aired on HBO on 7 December.

Litoff, a film editor, said: “Just working alone on the film was very painful, reading Ruth’s journals for the first time, seeing how much pain she was in for so many years was really hard for me to accept. I was grieving for the first time really. I hadn’t accepted that she was even dead.”

Ruth sought constant reassurance that she was loved, according to Hope. Her artwork ranged from large-scale images of flowers, nude self-portraits and street photography to dioramas and tchotchkes. Here, one of many pieces found in her apartment. Photograph: Hope Litoff

32 Pills explores the sisters’ relationships with each other and their parents, as well as Ruth’s struggles with mental illness. Despite much therapy, medication and support from family and friends, she attempted suicide 20 times from the age of 16.

As well as looking for answers in Ruth’s belongings, Hope talks to her sister’s friends and a former boyfriend, as well as a forensic pathologist about her death report.

Ruth, who was three years older than Hope, was found in her Manhattan loft in 2008, aged 42. She had decorated the apartment like a beautiful stage set with notes and gifts for close friends surrounding her. Some of her artwork, which included large-format images of flowers, nude self-portraits, collages and street photography, hung on the walls.

During filming Hope found Ruth’s plans for a show of her floral photographs at Bellevue hospital in New York, where she had psychiatric treatment. Hope arranges for this to take place. Titled Ruth’s Dream, the exhibition is to return to the hospital lobby later this year.

The Litoffs were a well-off but dysfunctional New York family. Their mother, who was preoccupied with her looks, found it hard to cope with Ruth’s mood swings, her father, described as “very 1950s Mad Men”, was a remote figure. Her mother died the year before Ruth’s death, and her father a couple of years after.

Hope said her mother and Ruth, who was diagnosed as having bipolar disorder, were very close. “My mom was the one who would always be running to my sister’s apartment. If the phone wasn’t answered, my mom would be there. She saved her many, many times. I think without that relationship, my sister just couldn’t make it on her own.”

Hope walks along the corridors of the New York storage facility where Ruth’s belongings were stored, on her way to look at them for the first time since her sister’s death six years earlier. Photograph: Hope Litoff

As a child, Hope idolised her creative, popular, beautiful, sporty big sister. “She taught me everything, about how to draw, what it means to be a good friend, how to shave my legs. She was the one who was there for me.

“She was incredibly magnetic and powerful, but she would also take over.”

It is clear in the film that Ruth dominated Hope both in life and in death.

Hope, 48, said: “I would like to be free from that. I had in my lifetime separated from her and that was a really tough thing to do and that was part of my getting sober the first time around.

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“I had to have my own life, I didn’t necessarily run to the hospital every suicide attempt. I thought now I’m married, I have children, I have to put my efforts somewhere else. I felt pretty good about that. It felt healthy and strong. But then once she committed suicide I thought I never should have done that. Making the film I felt that even more. I even felt that part of me that felt like destroying myself through drinking was in service of her, like somehow, she destroyed herself so I was going to destroy myself too. Of course, I’m an addict so there’s that element too, but it felt like the right thing to do.”

Footage of Hope ordering a double vodka in a bar after a police officer told her that Ruth had died in pain, is difficult viewing. It heralded a return to heavy drinking after 16 years of sobriety.

Hope said: “I shut my eyes every time I watch that.”

She started drinking in her teens to cope with the pressure of being the “good daughter” in contrast to Ruth and her troubles. Her mother would often ask her if she was happy, to which she felt she had to answer “yes”.

“When your mom basically says that you can’t have any feelings, which is how I interpreted ‘you’re happy, right?’ alcohol comes in very useful. I did it in secret so I kept my facade of being a good girl.”

In the film, Hope goes to rehab for 30 days. Since then she has returned to rehab twice and has been attending Alcoholics Anonymous.

“I’m sober now and feel thankful for every sober day. I’m so lucky that my husband’s still with me, I didn’t lose my children, my friends are still my friends, although they distanced themselves. It was tough.”

Hope lines up hundreds of bottles of pills that Ruth had been prescribed over the years during her treatment for mental illness. Photograph: Hope Litoff

Her husband, Todd, has watched 32 Pills once on his computer, but he refuses to go to any of the film festivals. “He doesn’t want to see the film, it’s too painful for him and I’ve got to respect that.”

Despite the toll it took on her and her family, she does not regret making the documentary. The “hardest part was realising how much Ruth was really suffering”, she said, but it has made her feel less guilty and helped resolve her conflicted relationship with her sister, moving from anger to compassion.

Sharing the film with her audience also helped her heal. “I can’t believe how many people come up to me crying and hug me saying: ‘The same thing happened to me’”, she said. “They’re helping me and I’m helping them. They all understand it’s never going to go away, which was my mission when I started the film. Now I know that it’s never going to go away and that’s OK. I can put it someplace else, it doesn’t have to destroy me.”

In the UK the Samaritans can be contacted on 116 123. In the US, the National Suicide Prevention Lifeline is 1-800-273-8255. In Australia, the crisis support service Lifeline is 13 11 14. Other international suicide helplines can be found at www.befrienders.org.